WASHINGTON - Comet ISON, the pristine comet from the vast reservoir of comets called the Oort Cloud, is headed for a close encounter with the Sun on Nov. 28 (Thanksgiving Day) at 2 p.m. EST. Each passing hour brings the comet inexorably closer to the Sun due to the pull of the Sun's gravity. The ultimate fate of the comet may well be determined by its solar rendezvous but as the well-known phrase of cometary studies goes: Comets are like cats; no one knows what they will do.
At its closest approach to the Sun, Comet ISON will pass only 730,000 miles from our star and will be traveling at 864,000 miles per hour. The comet will be subjected to temperatures of 5,000 degrees Fahrenheit and gravitational forces that could tear the comet apart. No one knows Comet ISON's fate, but astronomers around the world will watch and wait to see.
Comet ISON was discovered last year, and because it was so far away when sighted, there was some media speculation that it might become the "Comet of the Century," "bright as the Full Moon" and "visible in daylight." As time progressed, the comet did not live up to brightness predictions and the real possibility of a cometary dud for the viewing public arose.
ISON has come to life in the past week and brightened to naked-eye visibility, although only if you know precisely where to look. I tried to look for the comet while on a ship crossing the Atlantic Ocean but could not see it.
The comet's tail, caused by the sublimation of gases and dust from the comet's solid nucleus due to solar heating, has become quite a sight in photographs. The tail has lengthened considerably in the sky and developed complex structure.
It's diving into the glare of the rising Sun and will probably be no longer visible to ground-based observers after the 25th or 26th. The good news is, a fleet of solar spacecraft will be watching Comet ISON as it approaches and recedes from the Sun, so the world will have nearly a "live as it happens" view.
Comet ISON extremely interesting to the astronomical community because it has never been to the inner solar system before and it will pass so close to the Sun. These factors combine to provide an opportunity to observe and learn about a comet that is an unaltered leftover from the formation of our solar system 4.5 billion years ago. The close solar encounter will provide information on the comet's structure and composition, which will complement the myriad observations already made.
Ground- and space-based assets have observed Comet ISON since its discovery. Even spacecraft at Mars observed the comet when it passed by the Red Planet last month. Hubble Space Telescope (HST) has observed that the comet has not fragmented, as some scientists had predicted would happen before it passed by the Sun. Comet ISON is already being observed by NASA's STEREO solar spacecraft, with Solar Dynamics Observatory (SDO) and Solar Heliospheric Observatory (SOHO) waiting for the comet to enter their fields of view. A Japanese spacecraft will give us our best view when Comet ISON passes closest to the Sun.
No one knows what will happen to Comet ISON when it encounters the Sun up close and personal. We do know, however, that our knowledge of comets will be greatly enhanced by all of the data accumulated on Comet ISON. If the comet becomes a viewing spectacle or suffers a destructive end, you will read about it here at WTOP.
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